The present invention relates generally to plasma arc torches and, in particular, to dual gas plasma arc torches that utilize both a primary working gas and a secondary gas.
Plasma torches, also known as electric arc torches, are commonly used for cutting and welding metal workpieces by directing a plasma consisting of ionized gas particles toward the workpiece. In a typical plasma torch, a gas to be ionized is supplied to a lower end of the torch and flows past an electrode before exiting through an orifice in the torch tip. The electrode, which is a consumable part, has a relatively negative potential and operates as a cathode. The torch tip (nozzle) surrounds the electrode at the lower end of the torch in spaced relationship with the electrode and constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the electrode, an arc is caused to jump the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as an arc that extends externally off the tip. As the head or lower end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece because the impedance of the workpiece to ground is made lower than the impedance of the torch tip to ground. During this xe2x80x9ctransferred arcxe2x80x9d operation, the workpiece itself serves as the anode. A shield cap is typically secured on the torch body over the torch tip and electrode to complete assembly of the torch.
One type of conventional plasma torch is a dual gas torch in which a secondary gas flows through the torch concurrently with the primary working gas for purposes of cooling various parts of the torch or for affecting the plasma arc or the quality of the cut made in the workpiece. For example, it is common to direct the secondary gas flow onto the plasma arc as the arc exits the central orifice of the tip. However, this can lead to plasma arc instabilities, especially at low amperages, such as less than or equal to about 15 amps. These instabilities can adversely effect both the bevel angle of the cut and the surface quality of the cut.
The quality, such as surface finish, bevel angle and dross, of the cut made by the dual gas torch has been found to be a strong function of the composition of the secondary gas. A novel dual gas plasma arc torch is provided having a tip with bleed holes in fluid communication with both the primary and secondary gas flow paths in the torch so that a substantial portion of the primary working gas (e.g., oxygen) is bled off into the flow path of the secondary gas to form an oxygen rich secondary gas mixture in the torch. The size and number of the bleed holes regulates the amount of primary working gas bled into the secondary gas flow path. It is known that the optimal secondary gas mixture composition is a function of the current level at which the torch operates. Thus, the secondary gas mixture may be optimized for a particular torch by simply interchanging the tip with another tip having the desired number and size of bleed holes.
The torch of the present invention also incorporates a novel tip and shield cap design in which the shield cap sealingly engages the torch, and more particularly the tip, to prevent secondary gas mixture formed in the torch from impinging or otherwise being directed onto the plasma arc as the plasma exits the central orifice of the tip. Instead, the secondary gas mixture is exhausted from the torch through openings in the shield cap spaced radially from the central orifice to flood the kerf region of the cut with the oxygen (or other primary gas) enriched secondary gas mixture.
In general, a plasma arc torch of the present invention comprises a primary gas flow path in the torch for receiving a primary working gas and directing it through the torch to a central exit opening of the torch for exhaustion from the torch onto a workpiece in the form of an ionized plasma. A secondary gas flow path in the torch receives a secondary gas separate from the primary working gas and directs it through the torch. The primary gas flow path is in fluid communication with the secondary gas flow path substantially upstream of the central exit opening of the torch to bleed primary working gas in the primary gas flow path into the secondary gas flow path for admixture therewith to form a secondary gas mixture to be exhausted from the torch.
A tip of the present invention for use in a plasma arc torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch generally comprises an inner surface at least partially defining the primary gas flow path and an outer surface. At least one bleed hole extends from the inner surface to the outer surface for bleeding gas in the primary gas flow path into the secondary gas flow path for admixture with the secondary gas to form a secondary gas mixture. The at least one bleed hole is located in the tip such that admixture of the primary and secondary gases occurs generally within the torch.
A combination tip and shield cap of the present invention for use in a plasma arc torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch generally comprises the tip having a central exit orifice through which primary gas from the primary gas flow path exits in the form of an ionized plasma. The shield cap substantially surrounds the tip and has a central opening in generally coaxial relationship with the central exit orifice of the tip. The shield cap further has at least one secondary opening in spaced relationship with the central opening of the shield cap and in fluid communication with the secondary gas flow path for exhausting secondary gas in the secondary gas flow path from the torch. At least one of the tip and shield cap are configured for sealing the secondary gas flow path against fluid communication with the primary gas flow path intermediate the secondary opening and the central opening of the shield cap to prevent secondary gas in the secondary gas flow path from impinging on the primary gas as the primary gas exits the torch.
In another embodiment, a gas mixture system of the present invention for a plasma torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch generally comprises a plurality of tips each adapted for use in the plasma torch. Each tip comprises an inner surface at least partially defining the primary gas flow path and an outer surface. At least one bleed hole extends from the inner surface to the outer surface of each tip for bleeding gas in the primary gas flow path into the secondary gas flow path for admixture with the secondary gas to form a secondary gas mixture. The at least one bleed hole is located in the tip such that admixture of the primary and secondary gases occurs generally within the torch. The at least one bleed hole of each tip is sized such that the amount of primary gas bled from the primary gas flow path through the at least one bleed hole of each tip is different for each tip and corresponds to a current level.
A shield cap of the present invention for use with a plasma torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch comprises a hollow body having a central longitudinal axis. An upper end of the shield cap is adapted for connection to the torch, and a lower end has a central opening on said central longitudinal axis. At least one secondary opening is spaced radially outward from the central opening and is in fluid communication with the secondary gas flow path for exhausting gas in the secondary gas flow path from the torch. The shield cap has an annular sealing surface for sealing engagement with the torch to seal the secondary gas flow path against fluid communication with the primary gas flow path downstream of the fluid communication of the secondary opening with the secondary gas flow path to prevent gas in the secondary gas flow path from impinging on the primary gas as the primary gas exits the torch.
Finally, a method of present invention of operating a torch of the type having a primary gas flow path for directing a primary working gas through the torch and a secondary gas flow path for directing a secondary gas through the torch to cut a workpiece comprises directing primary gas to flow through the primary gas flow path to a central exit opening of the torch for exhaustion from the torch onto the workpiece in the form of an ionized plasma. Secondary gas is directed to flow through the secondary gas flow path of the torch. Primary working gas in the primary gas flow path is bled into the secondary gas flow path substantially upstream of the central exit opening of the torch for admixture with the secondary gas to form a secondary gas mixture to be exhausted from the torch generally toward the workpiece.
Among the several objects and features of the present invention is the provision of a plasma torch and method which increases the stability of the plasma arc; the provision of such a torch and method which improves the surface quality, dross and bevel angle of the cut made by the torch; the provision of such a torch and method which floods the kerf region of the cut with an oxygen enriched secondary gas mixture; the provision of such a torch and method which prevents secondary gas in the torch from impinging on the plasma arc as plasma exits the torch; and the provision of such a torch and method in which the secondary gas mixture is optimized for the current level at which the torch operates.
Other objects and features will be in part apparent and in part pointed out hereinafter.